Jan/Feb, 2024
www.us -
tech.com Fine-Pitch Direct Die Attach...
Continued from page 57 and SoICs.
Additional learning during
this work was in identifying trapped moisture within the polymer-based substrate as a cause for voids in the ACE dur- ing cure. These bubbles not only prevented connection in some cases but interfered with proper z-axis column formation. Pre - bake for the substrate was added as a step for this particular type of assembly. Two formulations were the
focus. These were the fine parti- cle ACE and the ultra fine parti- cle version. Stencil thicknesses of 0.001 to 0.005 in. (0.03 to 0.13 mm) were studied, as this tool has the most impact on estab- lishing bond line thickness. The target for choosing the best for- mulation, tool and bond line thickness was lowest average resistance values with lowest deviation among the 126 pads. Iterative testing was done. “Heat maps” based on the
126 pad locations were created to visually observe resistance val- ues within set target and accept- ance ranges. Ultimately the ultra fine particle ACE with a 0.001 in. (0.03 mm) thick stencil was chosen for this LGA-to-sub- strate assembly application.
Die-to-Die Bonding Fine-pitch die-to-die bond-
ing with the ACE was next. The development methodology was like the other projects. An initial focus was on measuring continu- ity and resistance at pad sites as part of identifying the optimum process parameters and stencil tool for this application. The degree of difficulty is greater with finer pitch. Dense arrays of 60 micron pitch die, with 30 micron pads and 30 micron spac- ing, were used. A Design of Experiments
(DoE) was established for the stencil studies. Laser profilome- try was used for 3D and 2D scans of the two surfaces to be bonded. All initial steps were done manu- ally: hand stencil printing, die placement with die bonder, batch oven cure and electrical probing. Prior to using fully function-
al devices, Quartz substrates patterned with the top layer of each die in the bond pair, were procured and used in early stud- ies. The purpose was to provide enhanced analysis of the bonded die pair before, during, and after bonding/curing has occurred. Bond parameters were observed at each step of the process: pre- bond, bond, and post alignment and cure. Due to the finer pitch
requirements, the test vehicles had a nickel layer applied during wafer fabrication, at the bond pad locations. Past experimenta- tion has shown nickel applica-
Reflo flow Soldering | Curing Soldering | Curing || Wave Soldering | S
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tion may lower the resistance of the bonded circuit by directing column formation to the metal- lized bond pad boundary. The nickel pads, acting as
localized magnets, attract col- umn formation during exposure to the magnetic pallet. This only applies to the bond pads them- selves, to concentrate the ferro-
magnetic metalized particles within the ACE more towards the connection points. This cre- ates a higher density of columns within each pad. Besides the addition of the
Cross-section of component attached and electrically connected with z-axis ACE.
nickel layer to the functional die, key parameter targets were updated for the project’s next phase. In the short-term align- ment, fiducials on the quartz plates were updated to improve bonding in X, Y, and theta; and the size of test probe pads were increased to improve accuracy and reduce testing time.
Continued on page 61
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